1. Field of the Invention
The present invention relates to methods of fabricating a layer of strained material that is then caused to become at least partially relaxed for applications such as electronics, optoelectronics and photovoltaics. Disclosed methods generally include providing of a structure that includes a strained material layer situated between a reflow layer and a stiffener layer, and applying a heat treatment that brings the reflow layer to a temperature equal to or greater than the glass transition temperature of the reflow layer. In other aspects, the invention also relates to fabricating semi-conductor devices from the layer of material that is at least partially relaxed.
2. Description of Related Art
When substrates are unavailable or are very expensive in solid form, they may be obtained in thinner layers by epitaxial growth on seed substrates. Nevertheless, the properties of these seed substrates are not always adapted to the materials from which one wishes to carry out the growth. In fact, seed substrates may present, for example, a thermal expansion coefficient and a lattice parameter that are different from those of the materials from which one wishes to carry out growth, which produces a certain number of defects in the layer formed, such as cracks that may develop during the growth or cooling of the structure, or the presence of lattice defects that reduce the efficacy of the devices formed later, or even the compression stress or tension stress of the layer.
Techniques for relaxing such strained material layers are known, particularly by introducing a reflow layer between the strained layer and a support substrate. But these techniques do not yield completely satisfactory results, as the strained layer is not always or completely relaxed elastically. The structure formed from a layer stack may also be degraded, and in order for the layer to relax, it may detach from the remainder of the structure. In addition, when the material is compression strained, this elastic relaxation may lead to buckling of the material, the roughness and amplitude between the peaks and valleys of the buckled layer then cannot be reconciled with the desired utilizations. When the material is tension strained, the relaxation often produces cracks and an increase in surface roughness.
The article “buckling suppression of SiGe islands on compliant substrates” by H. Yin et al. (in the Journal of Applied Physics, vol 94, number 10, published on Nov. 15, 2003) describes the elastic, lateral and buckling relaxations of compression stressed materials as implementing two competitive phenomena. According to this document, a first phenomenon consists of the lateral relaxation of the strained material; this relaxation then propagates from the edges of the film or islands formed in the film to the center of the film or island. It is thus explained that the smaller the island, the faster the lateral relaxation (which is accentuated by the thickness of the strained material layer). This lateral relaxation enables a film of substantially flat relaxed material to be obtained, with low surface roughness. For example, 60 micrometers×60 micrometers SiGe islands from epitaxy on an initial silicon substrate with a 30% Germanium content relax laterally and lead to obtaining a flat film whose roughness RMS is less than 2 nm.
As further explained in the aforementioned article, the second phenomena is relaxation by buckling, wherein the speed does not depend on the surface of the film or the island to be relaxed but rather on the stress in the material. Buckling leads to obtaining a film that is at least partially relaxed but very rough. It is possible that the film will fracture if the roughness exceeds a critical value. This phenomenon is particularly evidence in relatively thin films, which allow easy deformation, and thus, buckling.
In order to obtain a relaxed material with the best morphology, H. Yin recommends slowing down the buckling phenomenon and promoting the lateral relaxation phenomenon. To do this, he proposes depositing a layer of non-strained material on the film of material to be relaxed. This layer allows the total thickness of the material on the reflow layer to be increased by forming a bilayer (strained material layer and covering layer) and thus allows the lateral relaxation speed to be increased. The deposition of this covering layer also allows a bilayer structure that is mechanically more rigid with a lower propensity for curvature to be obtained. In addition, as the mean stress in the bilayer is lower due to the fact of the free deposition of the covering layer, the buckling force is lower. But, in terms of heat treatment, relaxation remains partial in the initially strained material. In fact, the relaxation is interrupted when the stresses are balanced in the bilayer. A multi-cycle method is then proposed in order to encourage lateral relaxation to the detriment of buckling relaxation. This is to carry out heat treatment on the bilayer until the relaxation allowed by the new stress balance is obtained, then the covering layer is reduced by a given thickness which allows a new stress balance and a new partial relaxation to be obtained at the end of the second relaxation annealing, while reducing the buckling relaxation phenomenon. These thinning/annealing steps are repeated until the covering layer is completely removed. The thickness of the covering layer to be removed may be identical at each cycle or may be variable and defined as being half of the thickness of the covering layer from the previous cycle. Optimization of the cycles combining these two variations is also planned but the relaxation method remains relatively long to put in place.